CHAPTER- 7 MOTION AND MEASUREMENT OF DISTANCES
Notes
1. In ancient times, people did not have any means of transportation. They traveled on foot; later, humans used animals for transportation. Boats were used for traveling through waterways.
- Until the beginning of the nineteenth century, people still depended on animals, boats, and ships for transportation from one place to another.
2. Measurement means comparing an unknown quantity with a known quantity of the same kind. This known fixed quantity is called a unit.
- The result of a measurement is expressed in two parts. One part is a number, and the other part is the unit of the measurement taken.
3. We need some units of measurement that are the same for all people.
4. In ancient times, the length of a foot, the width of a finger, and the distance of a step were commonly used as units of measurement.
5. In different parts of the world, people used the foot as a unit of length. The length of the foot varied slightly from region to region.
6. In the year 1790, the French developed a standard system of measurement called the metric system.
7. The system of units that is used nowadays is called the International System of Units (SI units).
- The SI unit of length is the meter.
- Each meter (m) is divided into 100 equal parts, which are called centimeters (cm).
- One centimeter is divided into ten equal parts, which are called millimeters (mm).
- 1 cm= 100 cm, 1 cm= 10 mm.
- The meter is not a convenient unit for measuring long distances. For this, we define a larger unit called the kilometer (km). 1 km = 1000 m.
8. We cannot directly measure the length of a curved line using a meter scale. To measure the length of a curved line, we can use a thread.
9. When an object moves along a straight line, this type of motion is called linear (straight-line) motion.
10. When an object continuously moves in a circular path, this type of motion is called circular motion. Example: the second hand of a clock.
11. In some cases, an object repeats its motion after a fixed interval of time. This type of motion is called periodic motion.
12. A ball rolls on the floor and moves forward while rotating. In this way, the ball performs rotational motion along with linear motion.
13. A change in the position of an object with time is called motion.
EXERCISE
Question- 1. Give two examples each, of modes of transport used on land, water and air.
Answer- Land transport – bike, car.
Water transport – boat, ship.
Air transport – aeroplane, helicopter.
Question- 2. Fill in the blanks:
(1) One metre is ______ cm.
(2) Five kilometre is ________ m.
(3) Motion of a child on a swing is ______.
(4) Motion of the needle of a sewing machine is ______.
(5) Motion of wheel of a bicycle is ________.
Answer- (1) One metre is 100 cm.
(2) Five kilometre is 5000 m.
(3) Motion of a child on a swing is Periodic.
(4) Motion of the needle of a sewing machine is Periodic.
(5) Motion of wheel of a bicycle is Circular.
Question- 3. Why can a pace or a footstep not be used as a standard unit of length?
Answer- The foot or step is not used as a standard unit of length because the size of a foot or step differs from person to person, which would cause confusion when different people measure length.
Question- 4. Arrange the following lengths in their increasing magnitude:
1 metre, 1 centimetre, 1 Kilometre, 1 Millimetre
Answer- 1 Millimetre, 1 centimetre, 1 metre, 1 Kilometre
Question- 5. The height of a person is 1.65 m. Express it into cm and mm.
Answer- We know that:
1 meter = 100 centimeters and 1 meter = 1000 millimeters
1.65 meters = 1.65 × 100 centimeters = 165 centimeters
1.65 meters = 1.65 × 1000 millimeters = 1650 millimeters
Question- 6. The distance between Radha's home and her school is 3250 m. Express this distance into Km.
Answer- We know that:
1 meter = 1/1000 kilometer
3250 meters = 3250/1000 kilometers = 3.25 kilometers.
Question- 7. While measuring the length of a knitting needle, the reading of the scale at one end is 3.0 cm and at the other end is 33.1 cm. What is the length of the needle?
Answer- Length of the needle = 33.1 centimeters − 3.0 centimeters = 30.1 centimeters.
Question- 8. Write the similarities and differences between the motion of a bicycle and a ceiling fan that has been switched on.
Answer- Similarity– Both have periodic motion.
Difference– A bicycle shows rotational motion along with linear motion, whereas a ceiling fan shows circular motion.
Question- 9. Why would you not like to use a measuring tape made of an elastic material like rubber to measure distance? What would be some of the problems you would meet in telling someone about a distance you measured with such a tape?
Answer- A flexible tape can stretch, so it may show the same measurement for different lengths. Therefore, we cannot use a flexible tape for measurement. While measuring distance, we would have to specify how much the tape was stretched. Hence, it can lead to incorrect measurements.
Question- 10. Give two examples of periodic motion.
Answer- Two examples of periodic motion- rotation of the Earth, vibration of a guitar string.